It is known that modern aircrafts, particularly civil or military airplanes, are equipped with aerodynamic control surfaces (for example leading edge slats and trailing edge flaps arranged on the wings) which are able to have a predetermined high-lift take-off position, selected among several separate take-off positions (generally three). Each take-off position corresponds to predetermined deflections of the aerodynamic surfaces at predetermined deflection angles.
In addition, it is known that, before an aircraft take-off, the take-off performance is calculated according to the following steps:
determining recommended values of the following speed parameters:                the decision speed V1: before the aircraft reaches the speed V1, the pilots can decide to stop take-off while being sure that he can stop the aircraft before the end of the runway;        the rotation speed VR, which corresponds to the speed at which an aircraft can lift its nose to take-off (that is it will have enough lift to tilt and fly without stall); and        the initial rate of climb V2, which is defined as the safety speed to be maintained during the initial climb phase; and        
selecting the take-off position to be applied to the aerodynamic surfaces of said aircraft upon take-off.
Determining recommended values of speed parameters V1, VR and V2, as well as selecting the take-off positions to be applied, is performed in particular using a plurality of information relating to:
the aircraft (take-off mass);
the weather conditions (wind, rain on the runway, atmospheric pressure, outside temperature);
the runway characteristics (length, condition).
Further, during these preliminary steps, regulatory safety criteria should be taken into account.
In particular, the recommended value of the initial rate of climb V2 should, for example, respect three regulatory safety criteria defined according to the standards in force. It should particularly be:
at least equal to a first speed threshold, corresponding to 113% of the stall speed of the aircraft (or speed Vs1g), that is V2≧1.13.Vs1g;
at least equal to a second speed threshold, corresponding to 10% of the minimum control speed with a breakdown engine (or speed VMCA), that is V2≧1.1.VMCA; and
such that the minimum gradient of climb γ of the aircraft with a breakdown engine is at least equal to a predefined gradient threshold (as a function of the number of engines on the aircraft), that is for example γ≧2.4%, for a twin engine aircraft, and γ≧3%, for a four-engine aircraft.
The safety criterion relating to the minimum gradient of climb γ with a breakdown engine may be shown as a dominant criterion with respect to the two other above-mentioned safety criteria.
In this case, the drag of the aircraft becomes a determining parameter: by taking measures to reduce the drag strength, the initial gradient of climb of the aircraft during take-off can be increased, while maintaining a constant thrust of the engines.
In addition, in order to respect the safety criterion relating to the minimum gradient of climb γ, with a breakdown engine, it can be contemplated to increase the initial rate of climb V2 in order to reduce the drag of the aircraft. It is what is called in aeronautics a “speed extension”. This way, the deviation between the speed V2 and the first speed threshold (1.13.Vs1g) is increased.
However, the increase of speed V2 requires increasing the rolling distance down the runway, which can be unfeasible on a short length runway. Thus, the length of the runway can limit the possible speed extension, and therefore, the allowable take-off mass of the aircraft.
Besides, when the safety criterion relating to the minimum gradient of climb γ with a breakdown engine is a predominant criterion with respect to both other beforementioned safety criteria, it can be considered to select another predetermined take-off position resulting in a high-lift, but less than that associated with the preceding selected take-off position.
However, since the number of predetermined high-lift take-off positions is generally limited (for example to three as previously indicated), when the selected take-off position is that which generates the lowest lift among all the predetermined high-lift take-off positions, one solution in order to meet the safety criterion relating to the minimum gradient of climb γ with a breakdown engine is to increase speed V2, but with the runway length drawback, as discussed above.